During a continuous or semicontinuous casting process for metals or their alloys, such as during continuous casting of steel, a hot melt is supplied to a casting mould which is part of a mould. In this application, mould means a casting mould, in one or more parts, for forming a cast strand of melt supplied to the mould and water box beams arranged around the casting mould. The casting mould, which is cooled and open in both ends of the casting direction, usually comprises cooled copper plates but may be made from another material with suitable thermal, electrical, mechanical and magnetic properties. The task of the water box beam is partly to stiffen and support the copper plate and partly to cool it and to conduct a coolant, such as water, to the mould. To make possible variation of the dimensions of the cast strand, the water box beams and the copper plates included in the casting mould are movable along an axis which is perpendicular to the casting direction. In the casting mould, the melt is cooled and formed into a cast strand. When leaving the casting mould, the cast strand comprises a solidified self-supporting surface layer which surrounds a liquid core of non-solidified melt. If inflowing melt is allowed to flow in an uncontrolled manner into the casting mould, it will penetrate deep down into these non-solidified portions of the cast strand. This makes the separation of unwanted particles, contained in the melt difficult. In addition, the self-supporting surface layer is weakened, which increases the risk of melt breaking through the surface layer formed in the casting mould.
From, for example, Swedish patent specification SE-PS 436 251, it is known to generate, by means of magnetic-field generating and magnetic-field transmitting devices, one or more static or periodic low-frequency magnetic fields and to apply these to act in the path of the melt to brake and distribute the inflowing melt. The magnetic-field generating and magnetic-field transmitting means are usually referred to as magnetic brakes and are used to a large and increasing extent in continuous casting of steel, preferably in continuous casting of coarser steel blanks such as
slabs, that is, blanks with a large, essentially rectangular cross section, and
blooms, that is, blanks with a large, essentially square cross section.
However, the method and the devices can also be used in casting of smaller blanks, that is, billets, with a small, essentially square cross section, as well as in casting of non-ferrous melts, such as slabs and extrusion billets of aluminium and copper as well as alloys based on these metals in semicontinuous processes.
The cast strand is formed by cooling and forming the melt supplied to the casting mould in the casting mould and continuing the cooling after the cast strand has left the casting mould. The casting mould is open in both ends of the casting direction and comprises walls, which usually comprise four separate copper plates. The copper plates are cooled during the casting. The copper plates are each fixed to a water box beam. The task of the water box beam is partly to stiffen and support the copper plate and partly to cool it and to conduct a coolant such as water to the casting mould. To make possible variation of the dimensions of the cast strand, the water box beams and the copper plates are movable along an axis which is perpendicular to the casting direction. Magnetic brakes are used both during closed casting, that is, when melt is supplied to the casting mould through a casting pipe with an arbitrary number and arbitrarily directed openings of the casting pipe opening out into the melt below the meniscus, and during open casting, that is, when melt is supplied to the casting mould from a container, a ladle or tundish, by means of a free tapping jet which hits the meniscus.
According to Swedish patent specification SE 91 00 184-2, a magnetic brake comprises means for generating and transmitting a static or periodic low-frequency magnetic field to act on non-solidified portions of a cast-strand. The magnetic-field generating means are permanent magnets and/or electromagnets, that is, coils with magnetic cores supplied with current. These magnetic-field generating means will hereinafter in this application be referred to as magnets. A magnetic brake comprises, in addition to magnets and cores, also magnetic return paths which close the magnetic circuits in which the magnets are arranged such that one or more closed magnetic circuits with flux balance are obtained close to a mould. These closed circuits comprise magnets, cores and a magnetic return path arranged close to the cores as well as a cast strand with melt present in the casting mould. One or more magnets are arranged on two opposite sides of the casting mould. In case of casting moulds with a rectangular cross section, the magnets are usually arranged along the long sides of the casting mould. Cores are arranged to transmit the magnetic field generated by the magnets to the casting mould and the cast strand present in the casting mould. According to the prior art, the magnets are placed outside the water box beam and must therefore be conducted through the water box beam by means of the core in order to reach the melt. According to the prior art, this is achieved with a core of magnetic material in one ore more pieces extending through the water box beam up to the wall of the casting mould. In those cases where energized electromagnets are used to generate the magnetic field, the coils of the magnets surround the magnetic core and are placed outside the water box beam.
In a continuous casting plant with a magnetic brake arranged and placed according to the prior art, the magnetic field is generated by magnets which are arranged outside the water box beams and is transmitted by means of cores to the casting mould. The length of the cores, which at least corresponds to the width of the water box beams, gives rise to magnetic losses. The losses, in turn, mean that the magnets have to be made larger. When using electromagnets supplied with current, this means that a higher electrical energy is needed to achieve the desired field strength in the melt. During the continuous casting, it is important that the melt does not adhere to the casting mould. For this reason, an oscillating motion in the casting direction is imparted to the casting mould during the casting by means of a shaking table, on which the casting mould, the water box beams and the magnetic brake rest. The larger the mass to be oscillated, the more energy is required. Therefore, it is desirable to limit the mass and the size of the casting mould, the water box beam and the magnetic brake. According to the prior art relating to magnetic brakes and to installation of magnetic brakes, at least the magnets and essential parts of the magnetic return paths are arranged outside the water box beams. In this way, it is difficult to obtain any significant reduction of the mass of the magnetic brake. Thus, it has not been possible to achieve the desired reduction of the required size and mass of a magnetic brake according to the prior art.
In addition, a possible frame structure, which is often arranged to support the casting mould and water box beams, must be further extended to provide space also for the parts of a magnetic brake which are arranged outside the water box beams.
One object of the invention is, therefore, to suggest a magnetic brake which has a reduced size and mass relative to electromagnetic brakes according to the prior art and an installation of this magnetic brake close to a mould which reduces the size and mass of the total installation while observing and fulfilling the metallurgical requirements for a magnetic brake. It is also an essential object of the present invention to reduce the total length of the cores included in the magnetic brake, whereby considerably less energy will be required both during oscillation of a casting mould with an associated electromagnetic brake and during magnetization of the magnets included in the electromagnetic brake.